<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let arr = [88,20,10,100,50]
    let re = -1
    for (let i = 0; i < arr.length; i++) {
      if (arr[i] === 10) {
        re = i
        break
      }

    } 
    document.write(`元素为10的下标为${re}`)
  </script>
 
</body>
</html>